Adhesives are widely used bonding materials that offer lightweight, high strength load bearing structures and can be used with a wide range of adherend materials such as metals, plastics, rubbers, composites and wood. Adhesives' applications can include bone repair procedures in orthopedics, bonded patch applications in airplanes, electronics packaging and building materials. Monitoring the quality of an adhesive bond is an essential procedure to ensure the safety of components in service. There are various mechanisms that lead to adhesive bonding degradation; such as moisture absorption, cracks, inclusions, wear, poor cure, and porosity. Numerous techniques exist for monitoring the quality of an adhesive bond, such as acoustic emission [1,2], radiography testing [3] and ultrasonic techniques. Ultrasonic techniques include normal and/or oblique incidence [4,5] and guided wave techniques, [6,7]. Guided wave techniques offer advantages such as confinement of the wave energy near the adhesive-adherend interface, which makes the wave highly sensitive to the interfacial mechanical properties and bonding conditions. In addition, guided waves propagate along the interface and can inspect large components much faster than with normal/oblique incidence methods.
Surgical implants play a major role in the lives of many people who experience serious injuries. Implants are man made devices that are “implanted” in the human body to replace, support and/or enhance biological components or structures in the body. Different kinds of implants can be inserted in the human body, which include knee, dental, hip and craniofacial implants such as nose, ear and eye. An important bonding process initiates at the prosthetic implant's surface after insertion. The bone tissue develops to form a strong bond with the implant surface (usually Titanium Oxide) and prevents relative motion. This process is called osseointegration and is an indicator of healing progression. Osseointegration was discovered by Brânemark in the 1950's, when he realized that rabbit bone could be permanently attached to titanium implants. It is defined as the formation of a direct contact between living bone and implant. This process allows the permanent fixation of the implant to the surrounding bone tissue. While osseointegration occurs with various types of prostheses focus will be on osseointegration of hip implants.
Total hip replacement (“THR”) is a surgical procedure adopted to replace a dysfunctional hip joint assembly. This procedure helps to a great extent to restore normal gait conditions to the patient and alleviate the pain due to a failed hip joint assembly. The hip joint consists of the femoral head, which is attached to the acetabulum to form a ball and socket arrangement. Deterioration of the hip joint could be caused by arthritis, which occurs with age due to degeneration of the articular cartilage. The wear of the articular cartilage causes bone to grind against bone; this causes severe pain, inhibits motion and eventually leads to bone fracture. Another cause is the significant reduction in bone density that leads to bone fracture and damage of the blood vessels. A common cause of hip joint deterioration among the young generation is injury due to extreme exercise.
The THR procedure is an intensive procedure, where the patient has to be completely sedated. The purpose of the operation is to replace a damaged hip joint assembly with a prosthetic implant. There are two commonly used approaches to ensure the formation of a strong bond between the implant surface and the bone. Either to use bone cement to enhance implant fixation i.e. cemented implant, or to use an un-cemented implant. In the latter case, the implant surface is coated with a porous layer to stimulate bone growth and the formation of a strong bond.
Post surgical complications are very common in THR procedures and patient follow-up is crucial. The most common type of complication is implant loosening, which occurs due to the bone re-modeling process that takes place after implant insertion. Remodeling takes place due to the changes in the loads transferred to the bone as a result of inserting an implant with a significantly different stiffness, which therefore shields the bone from the stresses normally transferred. This process leads to loss of bone mass and reduced bone density, which ultimately leads to implant loosening.
The bones in the wrists and ankles are considered to be short bones, while bones in the arms and legs, such as the femur are considered to be long bone. Bone is a complex structure. On the macroscopic scale it consists of two main layers; cortical and cancellous. Cortical bone is the compact outer layer that acts as a protective layer. Cancellous bone is the inner softer layer, which exists mainly in the end of long bone and within vertebrae. It is a porous structure formed of trabecular tissue. Although the cancellous bone is a soft tissue, the individual trabeculae are much stiffer than the bulk.
A wide range of values for the elastic modulus of single trabeculae have been determined. This variability is due to the differences and limitations in measurement techniques. The range of elastic modulus for single trabeculae is 1-20 GPa and the density is in the range of 1,600-2,600 kg/m3. The size of single trabeculae is in the range of 100-500 μm. On the other hand, the stiffness of the cancellous bone is lower than for single trabeculae. The range of values for the elastic modulus is 10-4,000 MPa and the density is 150-1,000 kg/m3. The elastic modulus is related to the apparent density (density of the trabecular structure and pores) through an empirically determined power law.
Various mechanisms exist for detecting osseointegration of hip implants. Imaging techniques such as X-Ray imaging, Dual Energy X-Ray Absorptiometry (“DEXA”) and Quantitative Computed Tomography (“q-CT”) are commonly used. Plain radiographs are widely used but have been shown to be highly inaccurate. It has been shown that unless a significant level of bone mineral density occurs; up to 70%, radiological signs will not be conclusive. DEXA, on the other hand, can provide a quantitative assessment of the bone mineral density; however, some unreliability exists since it depends on the exact positioning of the patients and errors would be introduced by patient movements. Quantitative CT-scans are widely used since they provide an accurate quantitative assessment of the bone mineral content; however its major drawback is the high radiation exposure.
Another approach is using vibration techniques. This approach can use sound waves in the audio range to excite femoral hip-implant assembly in vitro at different stages of cement curing. The results indicated that there was indeed an upward shift in the frequency response of the entire assembly. This approach has also been used to demonstrate that there is a shift in the natural frequency measurements of femurs with fixed and loose prostheses. Clinical studies have found that when loosening of the implant occurs, it can be detected by changes in the output signal. However, in an attempt to detect early stages of implant loosening, a study conducted on cadaver femora by simulating different stages of implant loosening and exciting the system with a sinusoidal force indicated that the system was performing well in detecting late stages of implant loosening but failed to identify early stages of implant loosening.
Further studies have investigated the accuracy of vibration detection techniques. Results were collected from vibration tests on a group of patients, as well as x-ray data for the same patients and were compared with each other. The results concluded that vibration testing was 20% more sensitive and diagnosed 13% more patients when compared with x-ray data.
A new generation of bio-implantable sensors is gaining momentum due to the major advances in the field of Micro-electro-mechanical Systems (MEMS). Implanting miniature sensors in the human body can be a major achievement. This would allow surgeons to monitor all parameters of interest in-vivo, which would lead to more tailored prescriptions, accurate assessments and early prediction of possible complications. In essence, each patient could become a biomechanics laboratory.
Various researchers have utilized bio-implantable MEMS sensors for in-vivo analyses. One has investigated the biocompatibility and wound healing behavior of bone tissue due to implanting a piezoresistive MEMS sensor in an animal spine. Results indicated healthy bone remodeling and no signs of inflammation or bone abnormalities. Another discussed the possibilities of using MEMS sensors in the spine and femur to measure fluid pressure.
The potential of bio-implantable sensors was also extended to the problem of implant loosening. Piezoresistive MEMS sensors have been utilized to measure the stresses at the bone implant interface in hip and knee implants respectively. Both approaches infer healing progression from the stress measurements since it is expected that the loads measured by the sensor will increase as healing progresses. In these approaches, values were assumed for the bone properties and the stresses were calculated accordingly.
It is, therefore, desirable, to provide an apparatus and method for characterizing adhesive bonding and osseointegration that overcomes the shortcomings in the prior art.